The present invention relates to mobile radio communication systems such as a wide-zone automobile radio telephone system, and more particularly, to such type of communication systems in which influences of communication faults can be prevented or restricted.
In a typical conventional mobile radio communication system of the above-mentioned type such as the one disclosed in U.S. Pat. Nos. 3,898,390 or 4,028,500, a composite service area is established as an assembly of small zones formed by service areas of a plurality of base stations. Accordingly, if a mobile unit located in a first small zone moves into an adjacent second small zone while communication is in progress, a radio signal transmission path extending to the mobile unit must be switched from a base station having the first small zone as its service area to the adjacent base station of the second small zone. This switching control is effected by a control station for controlling all the base stations and/or the mobile units through radio transmission paths having radio frequencies which are different for the respective base stations and wire transmission lines connecting the control station to the respective base stations. For transmission of the control signals, radio frequencies (or channels) which are different from each other are assigned to the respective base stations. Accordingly, even if a communication fault should arise on the control signal wire transmission line between the control station and the base station, the influence of the fault would be limited only to a small zone formed by one base station and would not extend to another small zone. However, in order to transmit these control signals, assigned frequencies equal in number to the base stations are necessitated as described above.
One approach to the solution of this difficulty, would be to assign one common radio frequency to all the control signals for all the base stations, but in that case the influence of the above-mentioned fault cannot be limited to a small zone of one base station and there is a fear that the influence may extend to small zones of other base stations. More particularly, if feeding of a control signal from the control station to one base station is interrupted because of a fault on a wire transmission line, an electromagnetic wave transmitted from that base station becomes unmodulated. Accordingly, the unmodulated carrier wave interferes with modulated carrier waves of the same frequency transmitted from all the other base stations, resulting in difficulty in the reception of the modulated carrier waves, that is, the reception of the control signals in the respective small zones to which these base stations belong.